Plasma arc furnaces are used in many industrial applications such as melting and heating masses of metal in the production of alloys. They are also used to heat and melt some industrial wastes in order to consolidate these wastes. Such an application of a plasma arc furnace is described in U.S. Pat. No. 5,731,564, issued Mar. 24, 1998 (Kujawa et al.).
Plasma torches used in these applications have metallic electrodes which are cooled with de-ionized water. These water cooled electrodes are the most vulnerable part of a plasma torch. Plasma arcs are initiated on these electrodes and the arc initiation regions are subject to relatively rapid erosion and wear. If erosion occurs at some concentrated area of the electrode, a leak will develop from the water cooling system. This type of failure causes very serious problems in a plasma arc furnace. It requires a shut-down of the furnace and a cooling of the entire system of molten materials and furnace. Only after cooling, can such a furnace be opened to gain access to the electrode so that it can be replaced. Electrode failures of this type result in shutdowns of furnaces which last for a number of days.
In prior art furnaces, it is necessary to break containment in order to change an electrode. If the furnace is being used to treat waste material containing toxic elements such as mercury, there is a need to perform expensive and complex purging operations before containment of the furnace is broken.
An electrode failure in prior art furnaces produces an even more severe problem when the waste materials being treated are radioactive. A cooling water leak into molten radioactive material produces vast amounts of radioactive steam which must be contained and treated. Additionally, there is a risk of an actual explosion. The risks associated with a possible leak has heretofore kept this form of plasma arc furnace waste treatment from being widely applied to radioactive materials.
This concern about cooling water leakage has produced frustration among those who seek effective methods for treating radioactive waste. A plasma arc furnace has a capability of treating waste in a highly contained environment. Additionally, plasma arc furnaces provide a method of reducing a great volume of waste into very compact and chemically stable rock-like objects. If the risk of cooling water leakage were to be eliminated, plasma arc furnaces would become a principal and very effective tool in the treatment of radioactive waste.
But even outside of the narrow field of radioactive waste treatment there is a need to eliminate the risk of cooling water leaks. Because such risks exist, normal operating procedures for plasma arc furnaces dictate that these furnaces be shut down periodically so that the electrodes can be replaced on a prophylactic basis. Because the life of an electrode is not entirely predictable, the interval of operation of a furnace between shutdowns is made relatively short. These shutdowns undermine the overall efficiency of plasma arc furnaces. Loss of efficiency appears to be a major factor in discouraging widespread acceptance of an otherwise promising furnace technology.
It is a goal of the present invention to provide a design for a plasma arc furnace with a reduced risk of cooling water leakage.
It is a further goal of the present invention to provide a plasma arc furnace which can be operated for long intervals without a need for shutdowns associated with torch electrode failure.
It is a still further goal of the present invention to provide a plasma arc furnace on which containment can be maintained while torch electrodes are replaced.